Receptacles made of polyester, for example bottles made of polyethylene terephthalate (PET), are conventionally manufactured in two successive stages: in a first stage, a thermoplastic preform is made by injecting one or more thermoplastic resins into a mold; in a second stage, the preform is blown and stretched in a blow mold so as to form the receptacle. These two stages are not necessarily performed immediately in line one after the other, and indeed they can be performed on different production sites. Under such circumstances, it is necessary to store the preforms on the site where they are produced so that they can subsequently be taken to the blowing machine which might possibly be situated on a different production site that is remote from the site where the preforms are produced.
A first known storage method consists in filling a container with the preforms in bulk. The container can be made of various materials (metal, card, . . . ), and can have a variety of shapes and sizes. The preforms are inserted from above, dropping under gravity into the inside of the open container. In order to reduce the mechanical impacts to which the preforms are subjected on being introduced into the container, special equipments have been developed so as to cause the height through which the preforms fall to be matched automatically relative to the-height of the pile of preforms that is building up inside the container, so as to limit the height of fall.
The major drawback of that method of bulk storage lies in poor use of the storage volume of the container, which amounts to low packing density, i.e. a small number of preforms being stored per unit volume. In practice, it is also observed that a convex dome forms at the top portion of the pile of preforms, where such a dome is characteristic of poor utilization of storage volume. However, for manifest economic reasons, associated mainly with the cost of transporting preforms, it is naturally preferable for storage volumes to be as small as possible, particularly for the purpose of making it possible to transport as large as possible a number of preforms in a given number of containers.
In order to improve the packing density of preforms in a container, proposals have already been made in the past to store preforms in ordered manner, by stacking preforms so that they are oriented in such a manner that the preforms in a given row point in the opposite direction to the preforms in an adjacent row. That solution is described in particular in United States patent U.S. Pat. No. 5,555,706. That solution does indeed enable the volume occupied by the preforms inside the container to be optimized in comparison with bulk storage. However, it presents the drawback of requiring special equipment that is complex and expensive in order to store the preforms in the container.
An object of the present invention is to provide a novel solution for storing preforms in a container which enables packing density (i.e. the number of preforms per unit volume) to be increased compared with the known method of bulk storage, but which is simpler to implement than the storage method described in above-cited patent U.S. Pat. No. 5,555,706.
This object is achieved by the method of the invention for storing preforms in a container. In a manner that is essential to the invention, the outside faces of the preforms are treated so as to make said outside faces more slippery, and the preforms are stored in bulk in the container, with the outside faces of the preforms after treatment presenting a coefficient of friction that is low enough to ensure that the packing density, i.e. the number of preforms stored per unit volume, is greater than the packing density that would be obtained under the same conditions with non-treated preforms.
The invention also provides: a method of manufacturing and storing thermoplastic preforms; apparatus for storing thermoplastic preforms; and an installation for manufacturing and storing thermoplastic preforms.